The major problem which hinders the use of linear peptides as drugs is related to the fact that such peptides exist in a rapid equilibrium between multiple conformations while only very few of these conformations are bioactive. This flexibility leads to poor selectivity, rapid proteolytic digestion and low bioavailability. One of the best ways to overcome this problem is by cyclization, which introduces conformational constraint into peptides.
In naturally occurring cyclic peptides, cyclization links together specific side chains and/or terminal groups in the peptide. These modes of cyclization, referred to as classical modes of cyclization, are highly limited due to the small number of amino acid side chains and peptide termini which lend themselves to cyclization. Thus, the diversity of possible constrained “classical” cyclic analogs of a given sequence is small.
Backbone cyclization overcomes these limitations (Afargan et al., Novel Long-Acting Somatostain Analog with Endocrine Selectivity: Potent Suppression of Growth Hormone But Not of Insulin, Endocrinology, 142:1 (2001) 477-486). Applying this technology, cyclization takes place by a covalent interconnection of one or more α-nitrogen(s) in the peptide backbone to one another or to the amino or carboxy termini or to a side chain.
Somatostatin is a cyclic tetradecapeptide found both in the central nervous system and in peripheral tissues. It was originally isolated from mammalian hypothalamus and identified as an important inhibitor of growth hormone secretion from the anterior pituitary. Its multiple biological activities include inhibition of the secretion of glucagon and insulin from the pancreas, regulation of most gut hormones and regulation of the release of other neurotransmitters involved in motor activity and cognitive processes throughout the central nervous system (See Lamberts, Endocrine Rev., 9:427, 1988). Additionally, somatostatin and its analogs are potentially useful antiproliferative agents for the treatment of various types of tumors.
In its natural form, somatostatin has limited use as a therapeutic agent since it exhibits two undesirable properties: poor bioavailability and short duration of action. For this reason, great efforts have been made during the last two decades to find somatostatin analogs that have superiority in either potency, biostability, duration of action or selectivity with regard to inhibition of the release of growth hormone, insulin or glucagon.
A group of somatostatin analogs (U.S. Pat. Nos. 4,310,518 and 4,235,886) includes Octreotide, the first approved somatostatin analog clinically available.
Another somatostain analog is veldoreotide (formerly known as PTR 3173 or DG3173), a conformationally-constrained, backbone-cyclic synthetic peptide, which is depicted in FIG. 1 and is as follows:                -γ-Abu-Phe-Trp-D-Trp-Lys-Thr-Phe-N-carbamoylmethyl        
where Abu is aminobutyryl.
Veldoreotide acetate is more commonly described as Cyclo(-γ-aminobutyryl-L-phenylalanyl-L-tryptophanyl-D-tryptophanyl-L-lysyl-L-threonyl-L-phenylalanyl-N-carbamoylmethyl-γ-aminobutyryl), acetate salt or Cyclo(-γ-Abu-Phe-Trp-D-Trp-Lys-Thr-Phe-N-carbamoylmethyl-γ-Abu), acetate salt.
Octreotide and veldoreotide have similar solubility in distilled water for injection, but very different solubility under physiological conditions. Once veldoreotide acetate progressed to human studies, it was found that, although veldoreotide acetate has the same receptor affinity for GH inhibition as Octreotide, an increased dose was required for the same pharmacological effect, when both were injected in lactic acid buffer. Upon investigating the cause of this problem, it was discovered that veldoreotide acetate is poorly soluble is physiological conditions, leading to “flip-flop” pharmacokinetics. Previous studies with veldoreotide acetate have found that injection of a solution of veldoreotide acetate results in adverse injection site reactions and limited bioavailability due to its relatively low solubility in isotonic solutions. Therefore, methods for enhancing the solubility of veldoreotide acetate in physiological (isotonic) solutions are needed.